layers... first start drawing the outline of her face, neck, eyes/nose/mouth/eyebrows... than, put another layer on by clicking a plus sign I think it is on the bottom right. Than start coloring, after you're done coloring erase layer 1 and voila! :P

Try using a reference image so that you can focus on details and Selena's features and face shape. http://ecx.images-amazon.com/images/I/51-FPULArzL._SL500_AA300_.jpg Theres an image of the album. If you want to you can use java so you can work with-